Information storage devices are used to retrieve and/or store data in computers and other consumer electronics devices. A magnetic hard disk drive is an example of an information storage device that includes one or more heads that can both read and write, but other information storage devices also include heads—sometimes including heads that cannot write.
The typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board (PCB) attached to a disk drive base of the HDA. Referring now to FIG. 1, the head disk assembly 10 includes at least one disk 11 (such as a magnetic disk, magneto-optical disk, or optical disk), and a track seeking/track following actuator 12, and at least one head gimbal assembly (HGA) that includes a head 14 and a suspension assembly 13. During operation of the disk drive, the track seeking/track following actuator 12 must rotate to position the head 14 adjacent desired information tracks on the disk 11. An arrow on disk 11 indicates the direction of disk rotation in FIG. 1. Track seeking/track following actuator 12 is typically a rotary actuator driven by a voice coil motor. The disk 11 includes a conventional information storage media (e.g. hard magnetic layer protected by a thin overcoat layer and supported by a substrate and various underlayers).
In a magnetic hard disk drive, the head typically comprises a body called a “slider” that carries a magnetic transducer on its trailing end. The magnetic transducer typically comprises a writer and a read element. The magnetic transducer's writer may be of a longitudinal or perpendicular design, and the read element of the magnetic transducer may be inductive or magnetoresistive. In a magnetic hard disk drive, the transducer is typically supported in very close proximity to the magnetic disk by a hydrodynamic air bearing. As the motor rotates the magnetic disk, the hydrodynamic air bearing is formed between an air bearing surface of the slider of the head, and a surface of the magnetic disk. The thickness of the air bearing at the location of the transducer is commonly referred to as “flying height.”
Magnetic hard disk drives are not the only type of information storage devices that have utilized air bearing sliders. For example, air bearing sliders have also been used in optical information storage devices to position a mirror and an objective lens for focusing laser light on the surface of disk media that is not necessarily magnetic.
The flying height is a key parameter that affects the performance of an information storage device. Accordingly, the nominal flying height is typically chosen as a careful compromise between each extreme in a classic engineering “trade-off.” If the flying height is too high, the ability of the transducer to write and/or read information to/from the disk surface is degraded. Therefore, reductions in flying height can facilitate desirable increases in the areal density of data stored on a disk surface. However, the air bearing between the slider and the disk surface can not be eliminated entirely because the air bearing serves to reduce friction and wear (between the slider and the disk surface) to an acceptable level. Excessive reduction in the nominal flying height degrades the tribological performance of the disk drive to the point where the disk drive's lifetime and reliability become unacceptable.
One way that a disk drive designer can improve the prospects of reaching an acceptable compromise in the “trade-off” described above, is to increase the complexity of the disk drive so as to dynamically control flying height. That is, additional head components and/or disk drive components that can function as a flying height actuator are included and actively controlled so that the flying height can be temporarily reduced only while the head is reading or writing. When the head is not reading or writing, it can “fly” at a slightly-higher nominal flying height to improve tribological performance. Such active control of flying height is sometimes referred to as “dynamic flying height” control (a.k.a. “DFH”).
Several types of flying height actuators have been published. One type of head-based flying height actuator incorporates a heating element into or near the magnetic transducer, to temporarily cause thermal expansion of a portion of the transducer and thereby bring it closer to the rotating disk. For example, U.S. Pat. No. 5,991,113 discloses such a resistive heating element, which can cause the pole tips of the transducer to protrude toward the disk relative to the air bearing surface of the slider such that the flying height at the location of the transducer is reduced. Additional examples of head-based flying height actuators employing a heater include U.S. Pat. No. 6,975,472, and U.S. Patent Application Publications US 2004/0184192 and US 2004/0130820.
If a transducer heater is used for flying height actuation via thermal expansion of the head near the transducer, such thermal expansion may also temporarily and locally change the contour of the air bearing surface in such a way that flying height is otherwise increased. That is, such temporary and local changes in the air bearing surface contour may undesirably oppose the intended effect of the transducer heater by increasing flying height when a further decrease is desired. This undesirable phenomenon may be referred to as thermal expansion “push back.”
Magnetostrictive material disposed in or adjacent to the magnetic transducer can also be configured to function as a head-based flying height actuator, by causing expansion or translation of all or a portion of the magnetic transducer towards/away from the disk surface. An example of a magnetostrictive flying height actuator is described in U.S. Patent Application Publication 2005/0243473.
Another head-based flying height actuation approach involves controlling the flying height via electrostatic forces, by applying a voltage between the slider and the disk. For example, head-based electrostatic flying height actuation is described in U.S. Pat. No. 6,359,746, U.S. Pat. No. 6,529,342, and U.S. Pat. No. 6,775,089.
Piezoelectric head-based flying height actuators have also been published, for example in U.S. Pat. No. 5,943,189, U.S. Pat. No. 6,501,606 and U.S. Pat. No. 6,577,466. Although in most cases the piezoelectric head-based flying height actuator functions by moving the magnetic transducer relative to the slider, the piezoelectric head-based flying height actuator may be used to change the flying height by altering a crown curvature of the slider (e.g. U.S. Pat. No. 6,624,984).
All of these head-based flying height actuators serve to change the flying height in response to an electrical input. Typically, the greater the electrical power applied to the head-based flying height actuator, the more the flying height will be reduced at the location of the transducer, until a portion of the head touches the disk surface—a condition known as “touch down”. Touch-down serves to limit further reductions in flying height, even if/when the electrical power applied to the flying height actuator is further increased. Actual or imminent touch down may even cause an increase in a time-average of flying height due to an increase in push back and/or an increase in flying height oscillations caused by contact forces.
It is generally not desirable to attempt to read or write data with a head while it is in a touch-down condition, because, for example, off-track motions and amplitude and frequency modulation of the read back signal, associated with frequent intermittent contact between the head and the disk, tend to degrade signal to noise ratio and increase error rate. Therefore, it is generally not desirable to attempt to read or write data with a head while its so-called “touch down power” (i.e. the power required to cause touch down) is applied to its head-based flying height actuator. However, the touch down power is, in general, unique to each head. Moreover, techniques to determine the touch down power for a given head have been unsuitable for a high-volume manufacturing environment, inconvenient, and/or have required specialized or expensive equipment. Thus, there is a need in the art for a practical method for defining a touch-down power for a head having a flying height actuator.